1. Field of the Invention
The present invention is related to a probe head for hyperpolarization and in general for multiple electromagnetic irradiation of a sample, the probe head acting as radiofrequency (RF) signal transducer, i.e. transducer intended both for generating a RF electromagnetic field suitable for the analysis of the sample by means of Nuclear Magnetic Resonance (NMR) spectroscopy, Magnetic Resonance Imaging (MRI) techniques, and Electron-Nuclear Double Resonance (ENDOR) technique in Electron Paramagnetic Resonance (EPR), the latter also known as Electron Spin Resonance (ESR), as well as for detecting a RF field signal generated by the sample in a way suitable for NMR and MRI techniques, the probe head also allowing a simultaneous, controlled, and efficient irradiation of the sample by microwaves (MW or μN) and higher frequency waves.
With the term hyperpolarization it is intended a kind of nuclear spin polarization of a material beyond the thermal equilibrium thereof. Known techniques of hyperpolarization are the Dynamic Nuclear Polarization (DNP), the Chemically Induced Dynamic Nuclear Polarization (CIDNP), the photo-CIDNP, to be used in the field of magnetic resonance techniques like NMR and MRI. The hyperpolarization techniques can enhance the sensitivity of the NMR and MRI measurements by orders of magnitude, opening the NMR and the MRI to unprecedented applications. Another example of multiple irradiation techniques in magnetic resonance is the ENDOR technique in EPR. In all the above techniques, the samples usually contain stable, chemically induced, or photo-generated paramagnetic species, as triplet states.
2. Description of the Prior Art
The NMR, MRI, and EPR techniques, developed since the middle of the last century, have had a huge impact thanks to their diagnostic capabilities in health analysis and to their ability to reveal the conformational details of complex bio-molecules like proteins.
A common trend in NMR, MRI, and EPR is the improvement in sensitivity and spectral resolution. In addition to magnetic resonance spectrometers working at higher and higher magnetic fields, in NMR and MRI several new probe heads have been developed, also in connection with hyperpolarization techniques as DNP. Analogously, in the field of EPR continuous efforts have been devoted to the development of efficient probe heads for multiple irradiation techniques as ENDOR. In this connection, different RF transducers have been proposed.
In particular, such RF transducers have been disclosed with the shape of solenoid coils or microcoils, in which the sample is accommodated inside the solenoid, as discussed for instance in (Webb, Progress in Nuclear Magnetic Resonance Spectroscopy vol. 31, 1-42 (1997)). However, as proposed so far, the solenoid is not suited for a controlled MW irradiation of the sample at high power, due to the distortion induced on the MW field in the sample region, which can generate a relevant dielectric heating of the sample. Moreover, it is not suited for large planar samples, which can be studied only increasing the size, thus sacrificing the RF sensitivity.
Then, a flat helical RF coil has been proposed in which the sample is placed on the device. It has a relatively poor RF field homogeneity, as discussed for instance in (Kentgens et al., The Journal of Chemical Physics vol. 128, 052202 (2008)). Moreover, it is not suited for a controlled MW irradiation of the sample at high power, since also in this case a strong distortion in the MW field distribution is expected around the metallic stripes edges.
Moreover, RF transducers in the form of striplines have been disclosed. The presence of a double ground plane shielding the central conductor of the stripline and the adjacent regions in which the sample can be usefully placed, make them unsuited for a simple and efficient MW irradiation of the sample at high power.
Further, RF transducers in the form of microstrips have been proposed. In the most sensitive versions, the RF magnetic field generated by the RF transducer is very inhomogeneous, limiting the application to extremely minute samples.
The US patent application No. Publ. 2011/0050225 (Prisner et al.) discloses a probe head for DNP-NMR and ENDOR techniques wherein the RF transducer is a RF resonator in the form of a single strip, at the same time such strip forming a portion of a corresponding MW resonator. This probe head is suited for thin samples, where it shows, however, a relatively modest RF filling factor, which translates in a limited sensitivity. For thick samples, it suffers of scarce RF field homogeneity.
The further US patent application No. Publ. 2012/0068706 (Prisner et al.) discloses a probe head for DNP-NMR and ENDOR techniques wherein the RF transducer is composed by a plurality of conductive strips, preferably connected in series in order to increase the RF conversion factor, at the same time such strips forming a portion of a corresponding MW resonator. This probe head is suited again for thin samples, where it shows, however, a relatively modest RF filling factor, which translates in a limited sensitivity, analogously to the previous case. For thick samples, it suffers of scarce RF field homogeneity.
In conclusion, none of the above cited probe heads and related devices can solve the numerous issues that influence, often simultaneously, the efficiency of the hyperpolarization and multiple irradiation techniques in magnetic resonance, as the sensitivity of the measurement; the dielectric heating of the sample; the compatibility with complex microfluidic structures and with large planar samples; the homogeneity of the RF irradiation; the efficiency and the control of the MW irradiation and the susceptibility broadening.